Propeller balancing mechanism



Nov. 14, 1944. MUELLER 2,362,842

PROPELLER BALANCING MECHANI SM Filed April 29, 1941 I ATTORNEY.

Patented Nov. 14, 1944 PROPELLER BALANCING MECHANISM I Robert K.Mueller, Erie, Pa., assignor to United Aircraft Corporation, EastHartford, Conn., a

corporation of Delaware Application April 29, 1941, Serial No. 390,897

3 Claims. (on. 7366) I This invention relates to an improved apparatusfor balancing rotating bodies such as airplane propellers and hasparticular reference to apparatusfor accurately determining theunbalance of an airplane propeller while mounted on the airplane anddriven by the airplane engine.

An object of the invention resides in the provision of an improvedapparatus for determining the location and amount of unbalance existingin a rotating body while the body is located in operative position anddriven by its usual power means.

' A further object resides in an improved apparatus for substantiallyeliminating propeller induced vibration from the structure of anairplane upon which the propeller is mounted.

Other objects and advantages will be more particularly pointed outhereinafter or will become apparent as the description proceeds.

Inthe accompanying drawing, in which like reference numerals are used todesignate similar parts'throughout, there is illustrated a suitableembodiment of an apparatus adapted to accomplish the balancinghereinabove referred to. The drawing, however, is for the purpose ofillustration only and is not to be taken as limiting or restricting theinvention since it will be apparent to those skilled in the art thatvarious changes in the illustrated embodiment may be resorted to withoutin any way exceeding the scope of the invention.

In the accompanying drawing, Fig. 1 is a diagrammatic illustration of anapparatus which may be utilized to determine the position and amount ofthe unbalance of air airplane propeller when the propeller isoperatively mounted on the airplane and driven by the airplane engine.Fig. 2 is a modification of the apparatus shown in Although methods havenow been developed by means of which an aeronautical propeller can befactory balanced at assembly to nearly perfect condition of static andaerodynamic balance, this initial condition does not necessarilycontinue for the service life such a propeller since propellers have nowbeen developed to a state of perfection at which they will render manyhundred hours of satisfactory service. During operation a certain amountof wear of the propeller parts and of erosion of the propeller bladesnecessarily takes place. The wear is mainly incident to pitch changingoperation of a controllable-pitch propeller while the erosion is mainlydue to grooving and other marring ofthe propeller blade surfaces byparticles which come in contact with the blades during landing andtake-off maneuvers of the airplane. As a noticeable scratch may lead toa serious concentration of stresses in the blade, it is customary ateach overhaul of the' propeller to refinish the blades to remove allgrooves and scratches from the surfaces thereof. This necessitates theremoval of a certain amount of material from the propeller blades and,while an efiort is made to remove the same amount of material atapproximately the same location from each of the blades of a propellerduring these overhaul operations, it is found that it is not possible tomaintain the propeller in perfect balance over a series of suchoverhauls and that because of this conditions and the above mentionedwear in the propeller parts, and in the propeller supportin bearings ofthe engine, sufiicient unbalance will accrue to enable the propeller toset up an objectionable vibration which is transmitted to the structuralparts of the airplane. Frequently some structural part of the airplanewill be in resonance with this propeller vibration at some operativespeed of the airplane, particularly at cruisin operation, and a veryobjectionable operating condition is thereby induced.

It is not considered practical to separately analyze the mass andaerodynamic unbalance components in the field to correct theseimperfections separately since the amount of total unbalance is notexcessive in any propeller that is considered fit to continue inoperation, and a correction of the total unbalance by the application ofsuitablecounterweights which will smooth out the propeller vibration atthe speed range used for the longest continued periods of operation,

siderations.

In correcting for propeller unbalance the vibration of some portion ofthe airplane structure, for example, the floor of the pilotscompartment, is calibrated against a known propeller unbalance andcorrelated with the rotationof the propeller and from this calibrationand correlation 'the amount and position of a corrective counterbalancefor the propeller may be determined. While the tests of propellerunbalance may convenientlybe made on the ground, it is quite possible toconduct these tests while the airplane is in flight so that a correctionat the most frequently used flight altitude can be obtained.

The apparatus for determining the magnitude and rotation of propellerunbalance will now be described in detail in connection with theaccompanying drawing in which the apparatus utilized is schematicallyillustrated.

In the accompanying drawing, the numeral I" generally indicates theaircraft which may be a single or multi-engine aircraft. The aircraftillustrated is of a multi-engine type and has a fuselage l2 containing apilots compartment l4 provided with a floor l6 and mounted upon a wingl8 which carries two or more engine nacelles, one of which is indicatedat 20, enclosing an engine, not illustrated, to which is operativelyconnected a propeller 22. An electrical pick-up, generally indicated at24, may be placed on the compartment floor IE to produce electricalimpulses in accordance with the vibrations transmitted to it through thefloor structure. This pick-up may have an armature 26 flexibly supportedin a casing 28 by suitable resilient means such as the spring 30 and acoil 32 mounted on a suitable core 34 firmly secured to the casing. Thisis known as a siesmic type of electrical pick-up and contains certainsuspension and damping features which act to stabilize the electricalimpulses produced by the pick-up in the operative range and avoid acondition of resonance between the natural frequency of the pick-up andthe vibration frequency within the range of vibrations for which thepickup is designed to operate.

A three phase electric alternating current generator, generallyindicated at 36, is mounted adjacent the engine-propeller combinationupon a suitable collapsible stand 38 adapted to be set up in front ofthe propeller when ground tests are being made and the generator isconnected with the propeller hub by a suitable coupling 40 having arelatively high degree of flexibility in angular directions so that thealignment of the generator shaft with the propeller shaft does not haveto be obtained with a high degree of accuracy. For flight tests thegenerator may be mounted as shown in Fig. 2 at some location on theengine 35 at which it may be driven at propeller speed.

The generator 36 has a two pole armature 42 rotatable with the propellerand a three phase field 44 connected to the three phase field 46 of anangularly adjustable transformer generally indicated at 48, remote fromsaid engine-propeller combination and generator to produce a rotatingelectrical effect in the field of the transformer. The core of thetransformer 48 is a single phase coil 50 manually rotatable relative tothe field 46 by means such as crank 41 and bevel gears 48 and so thatthe inductive action between the field and the core can take place atany selected angle of the core relative to the field. The angle of thecore relative to the field may be indicated on the stationary scale 55by the pointer 53 movable withthe core. By this means the effectiveangular position of the generator 36 relative to the propeller 22 can bemanually shifted electrically by the use of the remote controltransformer or phase shifter 48. The core 50 of the transformer 48 isconnected with the field coil 52 of an electric wattmeter usuallyadjacent said transformer and generally indicated at 54. This wattmeterhas a movable coil 56 mounted to swing in the field produced in the core58 by the field coil 52 and carrying a pointer 60 which swings over agraduated dial 62. The movable coil 56 is electrically connected withthe coil 32 of the pick-up 24. With this arrangement the pick-up willproduce current impulses in the coil 56 while the generator 36 willproduce current impulses in the coil 52. When these impulses are inphase a torque will be exerted on the movable coil proportional to theproduct of the current strength in the two coils and this torque willvary as the phase relation between the twotcoils varies being zero whenthe current impulses in the two coils are out of phase or ninety degreesapart in time relationship.

From this description it is at once apparent that if the pick-up 24produces an alternating current having a simple harmonic component atpropeller speed, the rotational position of the propeller at which thecurrent producing vibration occurs can be easily ascertained by rotatingthe transformer core 50 until the reading of the meter 54 is brought tozero, it being assumed of course that the angular relationship betweenthe core 50, the generator armature 42 and the propeller is known,having been fully ascertained at the time the generator is drivinglyconnected with the propeller. A zero reading of the wattmeter can beobtained regardless of any difference between the strength of thecurrent impulses generated by the pick-up 24 and the generator 36 sincea zero reading will occur in any case when the impulses from the twodevices are brought to a condition in which they are ninety degrees outof phase. The angular position of the core 50 at which this zero readingoccurs will indicate the angular position of the propeller at which thepeak voltages of the generator and the peak voltage of the pick-up areninety degrees out of phase. The wattmeter indication at a position ofthe core 50, ninety degrees from the above zero reading position in thedirec-- tion of positive meter indications will constitute a measure ofthe magnitude of the initial unbalance.

In actual practice the electrical pick-up 24 does not produce a simpleharmonic current alternating at propeller speed but does include acomponent of this nature in the complex current form generated by it.The remaining components of the pick-up output are, however,substantially nullified by the particular construction of the wattmeter54 the movable coil 56 of which has a large inertia and very high degreeof rotational damping so that it in effect, responds only to the torqueeffect of the component of the pick-up output which occurs insynchronism with the rotational speed of the propeller.

The magnitude of the propeller unbalance may be satisfactorilydetermined by the use of a trial weight of known mass which may beattached to the propeller at a known distance from the axis of rotation.

After the position of the core 50 for zero reading of the wattmeter 54has been ascertained, the core is rotated ninety degrees from thisposition and the reading of the wattmeter observed. The trial weight isthen attached to the propeller in a predetermined position and the core50 is rotated to obtain a zero reading of the wattmeter in the mannerindicated above and is then moved ninety degrees from that position andthe reading of the wattmeter is again observed. Having now measured theeffect of the original unbalance of the propeller as to both itsmagnitude and angular position with respect to the alternator and havingalso measured the combined effect of unbalance due to the originalunbalance plus the added trial weight, both as to relative position andmagnitude, it now becomes possible, by the use of appropriate algebraicor graphical methods, to subtract the effect of the original unbalancefrom that of the combined Displacement,

unbalance leaving as a result a measurement of the magnitude andlocation of the unbalance effect'caused by the addition of the trialweight.

Since the location and magnitude of the force unbalance produced by thisweight is known, the resultant reading of the wattmeter can now bereduced to desired units so that a certain number of graduations on thewattmeter dial'will represent a definite force unbalance acting on thepropeller and the direction of this force may also be determined. Havingnow calibrated the wattmeter to read in known units of unbalance, thetrial weight may be removed and a counterbalance weight added tothe'propeller as indicated tests that vibration, due to aerodynamicunbalance, returns when the altitude is varied fro that at which thebalancing test was made to particular flight altitude.

Inbalancing' the propellers of-a multi-engi e airplane, one propellercan be balanced. at a time and in flight testing the effect of another aknown weight is added to blade number I at a known distance from thepropeller'axis. It will now be possible to determine a new A and whichwill be appropriate to the combined unbalance now existing. Theprinciple of superposition applying to all linear systems, that is,those in which effect is proportional to cause, states that the effectof a combined unbalance like the one above is the same as' the sum ofthe effects of the component unbalances acting separately. Therefore, itshould be possible to determine an A1 and for the added unbalance alonewhich will then give us a relation between structural vibration at thechosen point and R. P. M., and the magnitude and angular position on thepropeller of any unbalance in the same plane. This information'will thenconstitute a fCalibrationf oi the airplane for the chosen point andR. P.M. and provides the basis for a simple determination of the unbalancewhich originally existed an'd'its subsequent correction. Fromtheprinciple of superposition:

3 Due to. Due to added combined Due to initial unbalance J unbalanceunbalance A cos (@4311) n cos (s-Help ul 005 was.

r Bysubstituting the trigonometric relation for the propeller may beeliminated by setting such other I propeller to run at a speed differentfrom the speed of the propeller being tested.

The following is a further explanation of the mathematics involved indetermining the actual magnitude of thepropeller unbalance, assumingthat the only vibration existing in the airplane is at the frequency ofpropeller rotation, at some point in the structure, say thecockpitfloor, the

motion has a certain time relation to the propeller angle. that is tosay, when the point is at its highest position blade number I may bethirty' degrees ahead of top center. Since the motion is assumed to besimple harmonic the point will be centered at one hundred and twentydegrees, in

lowest position at two hundred and ten degrees,

and centered again at three hundred degrees.

The displacement of the point (or its vertical component if othercomponents exist) maybe expressed as a1=A cos (wt+) where 21rXRPM t=timein seconds =30/57.3 radians=distance in radian traversed by blade numberi from top center before vibrating point of structure reaches highestposition ot=angle of blade number I in radians measured from top centerA=maximum amplitude of vibration As, the unbalance changes, both A and45 in this direction, and at a certain propeller R. P. M. due

to some initial unbalance in the propeller, and

V resorted to as come within cosine of the sum oftwo angles and thenfactoring out the coefiicients of cos wt and sin wt, an equation isdetermined wherein the coefficient of cos wt and-the coefficient of sinwt must individually be zero in order that this equation be satisfiedfor all values of at The two resulting equations maybesolvedsi'multaneously for A v and 41' to give the following formulas.

to calculate the initial unbalance from which the required correctionmay be found simply as a: mass diametrically opposite on the propellerand having the same moment. Thus, if

the initial unbalance must have had 71 times the moment of the added unbalence (measured in inch ounces or similar units) and must have beenlocated A radians from blade number I measured in the direction ofpropeller rotation. The required correction is a mass located so as tohave a moment of nA and at an angle of degrees ahead of blade number I.

While a suitable explanation of a suitable apparatus has beenhereinabove described and illustrated in the accompanying drawing forthe purposeof disclosing the invention, it is to be understood that theinvention is not limited to the particular apparatus so described andillusv trated/but that such changes in the parts of the apparatus andthe arrangement thereof may be the scope of the subjoined claims. j

Having now described the invention so that others skilled in the art mayclearly understand v the same, what it is desired to secure by LettersPatent is as follows:

1. In apparatus for balancing a rotating body including a device forproducing a series of electric current impulses when subjected tovibration caused by unbalance of said rotating body.

a multi-phase generator driven in timed relation with and locatedadjacent to said rotating body for producing another series of currentimpulses when rotated with said rotating body, and a two coil measuringdevice for indicating the in phase product of said two series of currentimpulses; control means remote from said rotating body and saidgenerator for shifting the angular relationship between the output ofsaid generator and said rotating body comprising a manually adjustablephase shifting transformer spaced from said rotating body and saidgenerator and having a multi-phase portion electrically connected withsaid generator and a manually rotatable single-phase portionelectrically connected with the field coil of said measuring device, andmeans for feeding said vibration induced current impulses to saidmeasuring device.

2. Apparatus for balancing an airplane enginepropeller combination whilethe propeller is in place on the airplane and driven by its own enginecomprising a device for generating a series of electrical impulses inaccordance with the vibratory movements of some portion of the airplanestructure, a polyphase generator located adjacent to and driven by theengine and producing a series of electrical impulses at the samefrequency as' the speed of rotation of said propeller and in a fixedphase relation thereto, a transformer located in a position remote fromsaid engine and propeller and generator, means for feeding the output ofsaid generator into said transformer, said transformer having means fortransforming the multiphase output of said generator into an output of aseries of single phase electrical impulses and having means for varyingand means for indicating the phase relationship between the output ofsaid transformer and said propeller, means for feeding the output ofsaid transformer and the first mentioned series of electrical impulsesinto a measuring device located adjacent said transformer said measuringdevice having means for measuring the product of any coincident portionsof the two series of electrical impulses fed thereto so that themagnitude and location of the propeller-engine unbalance may bedetermined by comparing the readings on the measuring device and theindications of the phase varying device respectively.

3. Apparatus for testing the balance of an airplane propeller while thepropeller is in place on the airplane and driven by its own enginecomprising, a pickup device for generating a series of electricalimpulses in response to the vibratory movements of some portion of theairplane structure, a polyphase generator located closely adjacent toand coupled with the propeller to be driven thereby for producing aseries of electrical impulses at the same frequency as the speed ofrotation of said propeller and in a fixed phase relation thereto, saidgenerator having a stationary polyphase winding, a phase-shiftingtransformer located at a position remote from said propeller andgenerator, said transformer having a stationary polyphase winding and amovable single-phase winding manually adjustable to alter the angularrelationship between it and the stationary winding, anelectrodynamometric indicating means having two coils angularly movablerelative to each other, electrical conductors connecting the movablewinding of the transformer with one coil of the indicating means and thepickup device with the other coil of the indicating means and alsoconnecting the stationary winding of the generator to the stationarywinding of the transformer, whereby the amount of the unbalance of thepropeller may be read from the indicating means when the manualadjustment of the phase shifting trans former produces a maximumresponse of said indicating means and the phase angle of the unbalanceis indicated by the position of the movable winding of the transformerwhen adjusted to cause a minimum response of the indicating means.

ROBERT K. MUELLER.

